Error while using if statement within for loop in javascript

I wrote a java script code to calculate driving distances along the 3 sides of a triangle, corners of which are formed by 3 zip codes. I want to do a simple calculation with the three distances such as a-b-c and display the result. I am getting null result. Where did my code go wrong?

function initialize() {
     var mapOptions = {
          zoom:      8,
          center:    new google.maps.LatLng(-34.397, 150.644),
          mapTypeId: google.maps.MapTypeId.ROADMAP
     };
     var map = new google.maps.Map(document.getElementById("map_canvas"), mapOptions);

     //INITIALIZE GLOBAL VARIABLES
     var zipCodesToLookup1 = new Array('31402', '30308', '30901', '31402');
     var output           = '<tr><th scope="col">From</th><th scope="col">To</th><th scope="col">Miles</th></tr>';
     var difference = "0";
     var totalDist = 0;
    // document.write(difference);
     //EXECUTE THE DISTANCE MATRIX QUERY
     var service = new google.maps.DistanceMatrixService();
     service.getDistanceMatrix({
          origins:      zipCodesToLookup1,
          destinations: zipCodesToLookup1,
          travelMode:   google.maps.TravelMode.DRIVING,
          unitSystem:   google.maps.UnitSystem.IMPERIAL
     }, function(response, status) {
          if(status == google.maps.DistanceMatrixStatus.OK) {
               var origins = response.originAddresses;
               var destinations = response.destinationAddresses;
               for(var i=0; i < origins.length-1; i++) {
                     var results = response.rows[i].elements;
                     output += '<tr><td>' + origins[i] + '</td><td>' + destinations[i+1] + '</td><td>' + results[i+1].distance.text + '</td></tr>';
                     if (var i = 0){
                     totalDist += results[i+1].distance.value;
                     }
                     else {
                     totalDist -= results[i+1].distance.value;
                     }

               }
              output += '<tr><td></td><td></td><td>'+(totalDist/1000*0.621371).toFixed(2)+ ' mi</td></tr>';


               document.getElementById('zip_code_output').innerHTML = '<table cellpadding="5">' + output + '</table>';
          }
     });
}

google.maps.event.addDomListener(window, "load", initialize);

html, body, #map_canvas {
    height: 500px;
    width: 500px;
    margin: 0px;
    padding: 0px
}

<div id="zip_code_output"></div>
<div id="map_canvas" style="border: 2px solid #3872ac;"></div>